The present invention relates to a continuous hot-dip plating apparatus and, more particularly to a continuous hot-dip plating apparatus having a roll assembly which has superior resistance to erosion caused by molten metal and also to wear caused by friction between the roll shaft and bearings.
In general, a roll is supported by metallic bearings. This applies also to the case of a roll of a hot-dip plating (or coating) apparatus or a hot dipping apparatus which is used in plating molten metal. Therefore, the shaft of a plating roll of a hot- dip plating apparatus, as well as bearings in support of the shaft, are provided with a layer of a material having a high resistance to erosion, e.g., stainless steel, high-chromium steel and so forth, formed by pad welding or provided as a sleeve. Unfortunately, however, even such an erosion resistant material erodes after prolonged use as a result of contact with the molten metal or the friction between the roll shaft and the bearings, with the result that the clearance between the roll shaft and the bearings is undesirably increased. Generally, the clearance between the roll shaft and the bearings grow to an unacceptable level within four days when used continuously in a plating bath of molten aluminum and within seven days when used in a plating bath of molten zinc. In consequence, vibration is generated as a result of rotation of the roll shaft, with the result that a uniform plating on the steel strips cannot be achieved.
In order to avoid this inconvenience, it has been necessary to temporarily suspend the plating operation to allow renewal of the roll shaft and the bearings. Such suspension of operation is undesirable and impairs the production efficiency, increases the production of unacceptable products due to the stoppage of the plating line, consumes much money and labor for the renewal of the roll shaft and the bearings, and so forth, with the result that the production cost is undesirably raised.
In order to reduce erosion and wear, various proposals have been made in, for example, Japanese Utility Model Unexamined Publication Nos. 60-35917 and 60-196029, as well as in Japanese Patent Unexamined Publication Nos. 62-205254 and 62-205255. On the other hand, Japanese Patent Unexamined Publication (called Nos. 60-298626 and 61-92320 disclose the use of a ceramic material as the material of the sliding surface layer of the bearings in a continuous hot-dip plating apparatus, to make an effective use of the multiplied advantages of ceramics: namely, erosion resistance and wear resistance. Japanese Patent Unexamined Publication Nos. 60-298626 and 61-92320 also disclose the use of ceramics as the surface materials of the inner and outer races and roller elements of a roller bearing which is intended for use under severe conditions such as high-temperature and corrosive conditions. The roller bearings disclosed in these patent specifications, however, employ impractically large numbers of parts and have to meet strict requirements for dimensional precision of the parts, resulting in a raised production cost. The coating of the rolling surfaces of the parts is met with a problem in that the thickness of the coating layer is undesirably limited. It is also to be noted that wear in the amount of several microns to ten or more microns can easily occur even when using ceramics. Thus, a considerably frequent renewal is also required when the rolling parts are coated with ceramics. In addition, the above-mentioned two patent specifications fail to disclose in what manner the rolling parts are coated with ceramics, as well as the selection, dimensions and evaluation of ceramics. Thus, these specification do not show any process for forming a roll bearings having a ceramics layer.